Nix: try disabling doctest the old way again

I thought the previous commit would be enough, as a similar change worked in
`ouroboros-network`. But the errors I'm seeing in CI seem to contradict that:

```
...
unpacking source archive /nix/store/96229sf9xxwixrarg8nz32r2z2w1lv7k-generic-data-0.9.1.0.tar.gz
source root is generic-data-0.9.1.0
setting SOURCE_DATE_EPOCH to timestamp 1000000000 of file generic-data-0.9.1.0/test/unit.hs
patching sources
building
Compiling package Setup.hs
[1 of 1] Compiling Main             ( Setup.hs, Setup.o )

Setup.hs:1:1: error:
    Could not find module `Distribution.Extra.Doctest'
    Perhaps you meant Distribution.Simple.Doctest (from Cabal-3.2.0.0)
    Use -v (or `:set -v` in ghci) to see a list of the files searched for.
  |
1 | import Distribution.Extra.Doctest (defaultMainWithDoctests)
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

builder for '/nix/store/3x12bvxzjln714qp4wwf94w79p6jy6i2-generic-data-0.9.1.0-setup.drv' failed with exit code 1
```

nix/haskell.nix

@@ -149,6 +149,14 @@ let
         packages.Win32.components.library.build-tools = lib.mkForce [];
         packages.terminal-size.components.library.build-tools = lib.mkForce [];
         packages.network.components.library.build-tools = lib.mkForce [];
+
+        # Disable cabal-doctest tests by turning off custom setups
+        packages.comonad.package.buildType = lib.mkForce "Simple";
+        packages.distributive.package.buildType = lib.mkForce "Simple";
+        packages.generic-data.package.buildType = lib.mkForce "Simple";
+        packages.nonempty-vector.package.buildType = lib.mkForce "Simple";
+        packages.semigroupoids.package.buildType = lib.mkForce "Simple";
+        packages.system-filepath.package.buildType = lib.mkForce "Simple";
       })
     ];
     # TODO add flags to packages (like cs-ledger) so we can turn off tests that will